For the manufacture of paste polyvinyl chloride, vinyl chloride is often polymerized according to the microsuspension process. According to this process, the vinyl chloride, together with the monomer-soluble radical-initiators and emulsifiers, is homogenized extensively in water prior to polymerization and then subjected to polymerization with or without stirring. The polyvinyl chloride latices so produced are not very stable and tend to coagulate and form grit. This problem of instability has become increasingly important as a result of more strigent regulations relating to environmental pollution since, following polymerization, so-called intense degassing operations have to be carried out to remove non-polymerized vinyl chloride from the latices. These degassing operations subject the latex to stress and often cause further coagulation.
The problem of instability can be solved by using emulsifiers having a better emulsifying action or higher concentrations of emulsifier. This, however, lowers the quality of the paste polyvinyl chloride. For example, the water absorption of the finished product is increased as a result of such measures. It is the emulsifiers having only a moderate emulsifying action, for example, alkali metal and ammonium salts of carboxylic acids, that result in excellent paste polyvinyl chloride products for the manufacture of especially transparent and water-resistant coatings.
A further attempt at solving the problem has been to use protective colloids in addition to the emulsifiers. Although this produces more stable latices, at the same time the quality of the polyvinyl chloride so obtained is impaired. Thus, for example, the viscosities of the resulting polyvinyl chloride pastes are thereby increased. Such increases in viscosity are not, however, desirable.